= Erythropoietin
<Erythropoietin> is a <glycoprotein> <hormone> that supports production of <red blood cells>, or <erythropoiesis>. It acts through the <erythropoietin receptor> on responsive precursor <cells>; the signal is not itself an <oxygen>-carrying substitute for <hemoglobin>. Experiments showed association of the receptor with <JAK2> and activation of that <protein kinase> after <erythropoietin> stimulation.<JAK2 associates with the erythropoietin receptor and is tyrosine phosphorylated and activated following stimulation with erythropoietin by B. A. Witthuhn and others>[Cell 74, pp. 227–236; Abstract, receptor association and kinase activation]{ref}
Purifying the <hormone> made its activity experimentally tractable. In 1977, researchers used seven separation steps on human urine, including <ion-exchange chromatography> and <gel filtration>, and reported $930$-fold purification with $21\%$ activity yield. The preparation had $70400$ activity units per milligram of <protein>.<Purification of human erythropoietin by T. Miyake and others>[Abstract; purification, specific activity and yield]{ref}
These quantities answer different questions. Specific activity asks how much biological effect remains per unit <protein> <mass>; yield asks how much starting activity survives purification. A preparation can become much purer while losing most of the original material.
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